The present invention relates to a method and an apparatus for measuring optical constants of a thin film, and in particular to a simple method for measuring them with a high precision in a non-destructive manner. It relates further to an optical integrated circuit or a semiconductor element fabricated by feeding back measured values of the optical constants obtained by the measuring method stated above to the fabrication process for making the thin film.
Heretofore, as methods for measuring optical constants of a thin film, there are well known 1). ellipsometry, 2) interference microscopic method, etc. At first, the ellipsometry has a drawback that for relatively thick films (.ltorsim.0.300 nm) the rafractive index cannot be determined quantitatively, unless the film thickness is known in some degree, because there exist periodical solutions in measured values. Further, measurement precision of the refractive index is in general as low as about 1.times.10.sup.3. Still further, this method has another drawback that it cannot be applied for measuring the distribution of the refractive index.
Next, for the interference microscopic method it is necessary to slice measured medium and in addition to polish it optically. Therefore a long time is required for preparing a sample therefor. Further, since it is a destructive examination method, it has still another drawback that the sample cannot be reproduced. Still further, in the case where a Michelson interferometer etc. are used, since a product of refractive index and film thickness, i.e. optical path is obtained as a measured value, this method has still another drawback that it is useful only under a presumption that either one of them is known.
On the other hand, recently, with developing research on the semiconductor laser, research is actively performed on the optical integrated circuit, in which light receiving/emitting elements such as semiconductor laser devices, etc. and various sorts of waveguide type optical elements are integrated on one substrate. One of the most basic constituent elements of this optical integrated circuit is an optical waveguide. As a method for measuring the effective refractive index of the optical waveguide the prism coupler method is generally widely known, as discussed in Applied Optics, Vol. 10, No. 11 (1971), pp. 2395-2413, etc.
In the prior art technique described above a prism having a refractive index n.sub.p is disposed closely to an optical waveguide, putting an air layer therebetween. Then a light beam is projected to the bottom face of the prism with a predetermined angle of .theta..sub.p to excite guided light by matching it in the phase with the optical waveguide as the guided light is taken out to the exterior by means of a second prism. In this case, the effective refractive index of the optical waveguide can be calculated according to the principle that the angle of the emerging light beam varies depending on the guided mode in the optical waveguide. However, by the present method, since the bottom surface of the prism is plane, the coupling efficiency is small and further, since at least 2 prisms are necessarily used, the S/N ratio is lowered. Therefore the present method has still another drawback that no measurement can be effected with a high precision. In addition, in the case where the refractive index of the optical waveguide varies continuously, it has still another drawback that it cannot be used. Furthermore, in the case where it is applied for measuring propagation loss of the optical waveguide, fluctuations in the coupling efficiency are great and therefore there is a problem that it is not practical.
Consequently there was known heretofore no method for measuring the refractive index, which is one of the basic parameters of the optical integrated circuit, with a high precision, and it as necessary to perform the optimization by trial and error in the course of the fabrication of elements.
Further, in fabrication steps of the semiconductor element, in order to assure the reliability of the semiconductor element, it is an extremely important problem to evaluate and control in-situ the physical properties of various sorts of dielectric thin films made of SiO.sub.2, Si.sub.3 N.sub.4, PSG, etc., which are used as chip passivation films or interlayer insulating layers in the multi-layered wiring, at forming the film.
Heretofore the main monitoring device used in thin film fabrication steps was a film thickness meter. Consequently there was a drawback that it is impossible to measure the physical properties at the film formation to feedback them for optimizing the film formation conditions.
In the present specification the term "thin film" should be interpreted in a broad sense. The thin film may be a conductive film (metallic thin film, etc.), a dielectric thin film (optical waveguide, glass, SiN, LiNbO.sub.3, etc.), a semiconductor thin film (Si, Ge, GaAs, etc.), or a macromolecular thin film (passivation film made of photosensitive resin, polyimide resin, etc.). The thin film may be formed by the sputtering method, the vacuum evaporation method, the CVD method the ion plating method, the thermal oxidation method, etc. The thin film may be a layer formed continuously on an original surface of a substrate by any method such as ion plating, etc. by using constituent materials working as an optical waveguide in an optical integrated circuit, a semiconductor element or an optical element.